It's bad practice to use SELECT * in live code, I'd go with something like this;

SELECT topics.Field1, topics.Field2, MAX(replies.createDate) createDate
FROM `chats_topics` AS topics
LEFT JOIN `chats_replies` AS replies
ON topics.id = replies.chatID
WHERE (".$search.")
AND (topics.id = replies.chatID)
AND (XXXX)
GROUP BY topics.Field1, topics.Field2
ORDER BY topics.createDate DESC

Add in your fields that you need to the select and also add them in the group by.